There are a range of different products options and choosing the right one is not always straight forward. It is important to set expectations upfront that there is no perfect solution to achieve one-way vision privacy in all lighting conditions. If the inside is illuminated and the outside is dark then the products will not work properly. Shining a powerful floodlight onto the outside surface of the window is a way to maintain privacy at night, however, this is not a realistic option in most situations. Otherwise blinds or curtains will normally be required to maintain privacy at night (sacrificing the view outside).
This page describes the main product options and their pros and cons. Contra Vision® Privacy Window Films™ offer a unique set of benefits which make them a great option in most but not all situations.